Catalytic cracking has reached a significant degree of sophistication, particularly in the field of controls systems. It is well known to use conditions such as the flue gas temperature or the difference between the flue gas temperature and dense bed temperature in a regenerator to control the cracker-regenerator-loop operation, e.g., by manipulating the flow of oxygen into the regenerator.
Basically in every catalytic cracking system there is a cracker-regenerator-loop in which the catalyst flows from the cracker to the regenerator and back. In the cracker the catalyst is contacted with hydrocarbon feedstock, such as a gas oil or a topped crude oil and coke is deposited on the catalyst during the cracking step. The quantity of coke depends among several factors upon the feedstock and the cracking conditions. The coke on the catalyst is the fuel for the regenerator. There the coke is burned, at least partially, from the catalyst. The flue gases leaving the regenerator are usually passed through cyclone separators to remove entrained solids (catalyst particles). The flue gases are very hot and a slight increase in temperature may cause damage to these cyclones.
An increase in the addition of air, or other free oxygen containing gases, to the regenerator will usually result in more combustion, particularly more after-burning in the dilute phase converting carbon monoxide to carbon dioxide. This increase in combustion results in a temperature increase. It is, however, not always true that increased air addition to a regenerator will result in an increased temperature of the dilute phase of the regenerator. It is therefore desirable to have a control system for a regenerator available that is flexible enough to operate in different and changing modes of combustion efficiently.